1. Field of the Invention
The present invention relates to a substrate support structure for clamping a substrate on a surface thereof, a clamp preparation unit configured for use of a substrate support structure, a lithography system comprising such a substrate support structure, and an arrangement comprising a clamp preparation unit and a plurality of such lithographic apparatus.
2. Description of the Related Art
Clamping of a substrate, for example a wafer, onto a surface of a substrate support structure, for example a wafer table, is well known in the semiconductor industry, and in particular in lithography systems. In such lithography systems, the clamped substrate is patterned by exposure to incident photons or charged particles such as ions or electrons. The clamping permits the realization of high precision patterning of a target portion of the substrate surface.
One method of clamping is by sucking away air between the substrate and the substrate support structure, i.e. by creating a vacuum in between them. However, if the clamped substrate is to be processed in a vacuum environment, this method will not be effective. Various other solutions exist for clamping a substrate in a vacuum environment, for example by means of electromechanical clamping. However, this solution is not suitable for use with charged particle lithography due to the undesirable influence of the electric fields used for clamping on the beams of charged particles.
In a lithography system, the clamping of a substrate to a substrate support structure should not only be such that the substrate maintains its position with respect to the substrate support structure during exposure. The thermal contact between substrate and substrate support structure should also be such that the heat load caused by the radiation on the substrate is effectively removed so that thermal contraction and expansion remain within specifications. This heat removal may be performed by a heat dissipation arrangement within the substrate support structure, e.g. by using a phase transition in a heat absorbing material that is brought into thermal contact with the substrate support structure surface as described in international application WO2008/013443 filed by the applicant.
The surface of a substrate support structure generally used in electrostatic clamp applications to hold the substrate typically is provided with a plurality of bumps called burls. The contact area between the substrate and these burls is limited. Consequently, in particular if swift removal of heat towards a heat dissipation arrangement is desired, the transport of heat from the substrate towards the heat dissipation arrangement may be a limiting factor.
International application WO2009/011574 filed by the Applicant, describes a lithography system with a substrate support structure for clamping a substrate by means of a liquid layer. The liquid layer is arranged to induce a capillary force such that the substrate is clamped on a surface of a substrate support structure. Adhesion of the liquid to the surfaces of the substrate on the one hand and the substrate support structure on the other hand creates a circumferentially extending liquid surface, concavely extending between the two surfaces. The so-formed concave liquid surface tends to maintain its shape, even if forces are applied to remove the substrate from the substrate support structure surface.
The liquid layer may further serve the purpose of enhancing the thermal contact between the substrate and the substrate support structure surface. Such enhanced thermal contact may enable the substrate to withstand higher heat loads without being subject to excessive contraction or expansion. This is of particular importance in applications such as lithography systems where there is a desire to realize higher throughput, i.e. in terms of wafers per hour. In such applications, the wafer is subjected to relatively high energy loads which normally translate into higher heat loads on the substrate.
However, the substrate clamping mechanism using a liquid clamping layer as described in WO2009/011574 will only be present for a limited period of time, e.g. due to evaporation and/or condensation processes. For many applications a longer clamping time period is desired.